Re: [BUG?] 'git rebase --abort' couldn't abort aborted rebase

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Fri, Feb 07, 2020 at 12:10:08PM +0100, SZEDER Gábor wrote:
> That's a good subject, isn't it? :)
> 
> So, to clarify: apparently it is possible to abort an ongoing 'git
> rebase' process with ctrl-C in just the right moment that a subsequent
> 'git rebase --abort' will refuse to clear it up.
> 
> I somehow messed up the upstream and branch parameters of 'git
> rebase', and ended up trying to rebase a fairly recent (post v2.24.0)
> branch on top of v2.22.0.  Upon seeing the unexpectedly large number
> of patches I realized that something is wrong, hit ctrl-C right away,
> and this is what happened:
> 
>   $ git rebase v2.22.0 <a-branch-on-top-of-2.24.0>
>   First, rewinding head to replay your work on top of it...
>   Generating patches: 100% (1108/1108), done.
>   Applying: send-email: move the read_config() function above getopts
>   Applying: send-email: rename the @bcclist variable for consistency
>   Applying: send-email: do defaults -> config -> getopt in that order
>   Using index info to reconstruct a base tree...
>   M       git-send-email.perl
>   M       t/t9001-send-email.sh
>   Falling back to patching base and 3-way merge...
>   Auto-merging t/t9001-send-email.sh
>   Auto-merging git-send-email.perl
>   ^C
>   ((5f07da12ac...) *|REBASE 3/1108)$ git rebase --abort 
>   error: could not read '/home/szeder/src/git/.git/worktrees/WT/rebase-apply/head-name': No such file or directory
>    
> "Fortunately" it was in a separate worktree, so I could easily get out
> of the situation by forcibly deleting that worktree.  Unfortunately,
> that was exactly what I did, instead of securing the failed state for
> later analysis...  sorry.

> All this is with a git built from current 'next', with a bunch of
> unrelated (none of them touches rebase or the sequencer) patches on
> top.

Trying to reproduce it is a hit or miss... well, mostly miss :)

There is a rather short window when 'git rebase' applies patches
before hitting a first merge conflict.  If the ctrl-C arrives before
'git rebase' starts applying patches, then it cleans everything up,
and we are not rebasing, so there is no need for 'git rebase --abort'.
Once 'git rebase' stops because of the merge conflict we get our shell
back, and 'git rebase --abort' works as it should.  But after a good
couple of tries I managed to hit ctrl-C while 'git rebase' was
applying patches:

  $ ./BUILDS/v2.25.0/bin/git rebase v2.22.0 v2.24.0^{commit}
  First, rewinding head to replay your work on top of it...
  Generating patches: 100% (1049/1049), done.
  Applying: send-email: move the read_config() function above getopts
  ^C
  ((v2.22.0) +|REBASE 1/1049)$ ./BUILDS/v2.25.0/bin/git rebase --abort
  error: could not read '.git/rebase-apply/head-name': No such file or directory

Note that I'm using v2.25.0 here, so the problem is not with 'next' or
with my patches.  Also note that I reproduced the error in the main
worktree, and only using commits that are present in git.git.


Finally, note the 'v2.24.0^{commit}' parameter, in particular the
'^{commit}' part.  That's important, because without it we stumble
upon _another_ bug:

  $ git rebase v2.22.0 v2.24.0
  error: Object 1cc4bc0fcd93f816d514d77c29f2cc9ffdd8ae09 not a commit
  First, rewinding head to replay your work on top of it...
  Generating patches: 100% (1049/1049), done.
  Applying: send-email: move the read_config() function above getopts
  < ... applying further patches and hitting a merge conflict ... >
  Resolve all conflicts manually, mark them as resolved with
  "git add/rm <conflicted_files>", then run "git rebase --continue".
  You can instead skip this commit: run "git rebase --skip".
  To abort and get back to the state before "git rebase", run "git rebase --abort".
  (detached HEAD *+|REBASE 7/1049)$ git rebase --abort 
  error: update_ref failed for ref 'HEAD': cannot update ref 'HEAD': trying to write non-commit object 1cc4bc0fcd93f816d514d77c29f2cc9ffdd8ae09 to branch 'HEAD'

So 'git rebase' shows an error right at the beginning when rebasing a
tag, but then continues anyway.  However, 'git rebase --abort' can't
restore the original state.